Dynamic nuclear polarization (DNP) is a technique that generates an excess of one nuclear spin relative to the other orientation. This excess can be on the order of several thousand-fold at cryogenic temperatures and several hundred thousand-fold at room temperature. This increase in population of one nuclear spin relative to the other is seen as an increase in the signal-to-noise ratio of measurements in nuclear magnetic resonance (NMR) systems such as magnetic resonance imagers (MRI).
To achieve high levels of polarization via DNP, materials or samples must be cooled to extremely low temperatures, often less than four Kelvin and optimally in the range of one Kelvin. These low temperatures are typically achieved by reducing the pressure above a volume of liquid helium. As the pressure above the helium bath is reduced the temperature of the bath is reduced as defined by the saturation curve of liquid helium. The introduction of warm samples into this environment can significantly impact the temperature of the helium bath as well as the polarization of any samples that are already present in the bath. Additionally, the process of cooling samples results in the vaporization of liquid helium from the helium bath, impacting the duration the helium bath can be maintained and the number of samples that can be processed.
A conventional means of reducing the pressure above a helium bath is the use of one or more mechanical pumps. These pumps expel helium into the ambient environment as a result of this pumping process, making it difficult and expensive to reuse the cryogen. The quantity of helium in this bath can be either static, being filled before the pumping is initiated, or dynamic through the use of a second helium reservoir connected to the pumped region via a regulated passageway such as a needle valve. The static system often exhibits a limited operational period due to size constraints of the helium bath. The dynamic system, although more flexible, contains mechanical components within the cryogenic environment, potentially limited the robustness of the device.
A sorption pump may be used in a closed cycle cryogenic system that is designed to generate one-Kelvin temperatures without loosing cryogen (liquid helium) volume. This sorption pump contains a charcoal-based sorbent that absorbs gaseous helium at low temperatures, thus acting as a means to reduce the pressure above a volume of liquid helium. When the temperature of the charcoal is elevated the gaseous helium is released from the sorption pump, thus acting as a source of helium for the liquid helium bath. This sorption pump may be operated in a cyclic fashion that first condenses liquid helium and then reabsorbs the helium while generating reduced temperatures. In this cyclic manner of operation the total volume of cryogen remains constant. The fact that this system operates without loosing cryogen volume is a significant benefit to ease of operation by eliminating the need for frequent cryogen transfers as well as a cost savings through the elimination of cryogen purchases.
However, a limitation of this system is that during the condensation portion of the sorption pump cycle, the volume of liquid helium generated is limited by several geometric considerations including the mass of charcoal, the amount of gaseous helium loaded into the charcoal and the physical size of the container into which the helium is condensed. Because of this limited volume of liquid helium the amount of heat directed to the helium bath directly impacts the amount of material or number of samples that can be cooled and polarized during one thermal cycle.
Accordingly, there is a need to improve upon current cooling systems and methodologies.